The gas access through a thin porous layer is studied in relation to the reactant gas transfer in the gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells. In the considered situation, the surface through which the gas enters into the porous layer is partially occluded. The aim is to design the porous medium microstructure so that the gas flux is uniformly distributed over the gas exit surface. The method combines pore network simulations and an optimisation method based on a genetic algorithm. The optimisation procedure is performed for a dry GDL. Then, various scenarios of wet GDL are considered in order to check whether the optimised GDL still performs better than the non-optimised GDL in the presence of liquid water.